Semiconductor devices that modulate transmitted light by passing the light through a multiple quantum well (MQW) structure are well known. An example is described in U.S. Pat. No. 4,833,511. In an MQW structure, relatively thin quantum well semiconductor layers are sandwiched between quantum barrier layers. The quantum barrier layers have larger energy band gaps than the quantum well layers. As a result, when the quantum well layers are sufficiently thin, typically ten nanometers or thinner, quantum mechanical effects are produced. Charge carriers can be confined to the wells and the population of excitons is increased significantly over bulk semiconductors. As a result, exciton resonances, alterations of effective band gaps, and other phenomena having quantum mechanical origins are produced. Some of these phenomena have been exploited in the known light modulators, including MQW structures.
FIG. 4 shows in schematic cross-section an MQW structure such as that used in known light modulators. Typically, these modulators have a pin-type structure including a p-type aluminum gallium arsenide (AlGaAs) layer 1 and an n-type AlGaAs layer 2 sandwiching the alternating quantum well and quantum barrier layers that form the MQW structure 3. Incoming light 4 passes through cladding layer 1, the MQW structure 3, where the modulation takes place, and out through cladding layer 2 as modulated light beam 5. As indicated in FIG. 4, the MQW structure 3 includes a plurality of pairs of quantum well layers 31 and quantum barrier layers 32. The reference numbers employed in FIG. 4 include a prefix indicating the type of layer, i.e., well or barrier, and a suffix indicating the respective layer pair. The structure of FIG. 3 includes fifty such pairs of well and barrier layers that are usually semi-insulating and each about ten nanometers in thickness. Thus, the MQW structure 3 is essentially an intrinsic-type body so that the light modulator has a pin structure. The incident and modulated light is generally perpendicular to the well and barrier layers 31 and 32.
The structure of FIG. 4 is typically prepared by growing the layers 31 and 32 by molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), or some other process that allows stringent control of layer thicknesses. Since each layer is only about twenty atomic layers thick, extremely precise process control is required to grow the alternating layers with the desired thicknesses.
Several light modulating mechanisms may be exploited with the MQW structure. An MQW structure is well known to exhibit one or more exciton resonances. At the wavelengths of those resonances, light absorption increases sharply. When the wavelength of the light being modulated is coincident with one of the exciton absorption peaks, a large portion of the incident 4 is absorbed and does not emerge from the modulator as light beam 5. The exciton absorption peaks shift in wavelength when the modulator structure of FIG. 4 is reverse-biased with an electrical voltage. With incident light 4 of a wavelength near the exciton absorption peak, variation of the reverse bias voltage applied across the structure varies the wavelength of the absorption peak and the degree of light absorption, thereby modulating the light beam 5 emerging from the modulator.
While the modulator of FIG. 4 is effective in modulating incident light of moderate intensity having a wavelength at the exciton absorption peak, the modulator is easily overloaded when the intensity of the incident light becomes too large. In that situation, only some of the incident light is modulated and the remainder of the light passes through the modulator without being modulated. Moreover, the precise wavelength of the exciton absorption peak varies not only with the bias voltage but also as a function of the temperature of the modulator, a temperature that can vary depending on the intensity of the incident light as well as the environmental temperature. The temperature dependence of the modulator can be mitigated, but not eliminated, by modulating light having a wavelength on the longer wavelength side of the exciton absorption peak.
In addition to exploitation of the exciton absorption phenomenon, the Quantum Stark Effect and inherent electro-optic effect of the semiconductor material may be exploited to modulate transmitted light. In both of these effects, the refractive index of the semiconductor materials is altered in response to the electric field produced in the modulator of FIG. 4 when an electrical signal is applied across layers 1 and 2. However, the degree of modulation achieved with these effects is relatively small, particularly since the thickness of the modulator through which the light transits is relatively small. If the thickness of the structure is increased to increase the effect of the refractive index change, an undesirable amount of the incident light is absorbed.
Accordingly, it is desirable to provide a semiconductor light modulator that produces a large degree of modulation of incident light, that is relatively free of temperature variations, that can modulate light of a desired wavelength not related to an exciton absorption peak, and that can be relatively easily produced, i.e., without the stringent process control required to produce an MQW structure.